This application claims foreign priority benefits under 35 U.S.C. xc2xa7119(a)-(d) from German patent application ser. no. P 103 07 933.5 filed Feb. 25, 2003.
The present invention relates to a calibration device for calibrating oven temperatures.
Ovens of the type commonly referred to as retort ovens are required for the production of dental ceramics. In the operation of such ovens, a pivotable oven hood is lowered in order to effect the uniform heating of the product to be heated. Rapid access to the heat treated product is made available via raising of the hood, so that this approach has proven its value.
The quality of the produced heat treated product, which may be, for example, in the form of a dental ceramic, is strongly dependent upon precise adherence to the prescribed heating curvexe2x80x94i.e, precise following of the temperature path. In this connection, the oven typically has a temperature sensor so that a control of the temperature is possible.
To be sure, the quality of the temperature path-following process is, naturally, only as good as the precision of the temperature sensor. In this connection, such retort ovens must be regularly re-adjusted and calibrated.
Moreover, with respect to such ovens deployed for dental product treatment, an exact calibration must be regularly undertaken with respect to numerous types of such ovens.
Numerous different approaches are known for effecting a calibration via simple means. For example, DE-OS 42 06 540 discloses an oven in which two thermo-elements are to be deployed in order to effect a calibration. Such thermo-elements are, to be sure, not precise.
Moreover, it is known from DE-OS 100 08 603, to use the melting points of gold and silver calibration wires as the calibration points. In this approach, a melt wire is provided whose melting interrupts a circuit, whereupon the melting temperature can be precisely captured or registered via a circuit a portion of which comprises the melt wire operating in the circuit as the calibration element.
Numerous other solutions are known in which at least two temperature points are to be registered in connection with a calibration. The suggested approaches are, however, comparatively expensive while nonetheless not being especially precise, in fact, in exactly those instances in which the melting wires are used as an opening contact or if a meltable metal is to actuate a closing contact. In both instances, the cohesion and adhesion of the melted metal must be accommodated, as such can exert an unfavorable influence on the contact opening process as well as the contact closing process.
It is no coincidence that the classical substances for temperature control are noble metals such as gold and silver. Aside from the benefit of having exactly defined melting points, these characteristics of such metals including, in particular, their chemical noble character, impart the property to these metals of having no tendency to form chemical compounds with the surrounding environmental atmosphere including, in particular, oxides.
In contrast, normal metals form top surface oxides at the high temperatures of a temperature calibration process, with the melting points of such top surface oxides usually lying well above the melting point of the metal so that the metal is effectively disposed in a significantly mechanically constrained and electrically insulated encasement, which considerably hinders establishing contact with the metal or effecting loss of contact with the metal.
These metal melt performance characteristics are even more disadvantageous in view of the fact that the noble metals comprise comparatively high melting points so that a calibration at low temperatures in, for example, the range of 600xc2x0 C., should preferably be undertaken with the use of base metals such as, for example, aluminum. Aluminum would be fundamentally suitable for an exact temperature calibration, as the melting point of pure aluminum can be defined very preciselyxe2x80x94namely, to 3 digit positions to the right of the decimal point. However, aluminum is a highly reactive metal whose use-beneficial characteristics reside in the fact that, even at room temperature, a protective layer of aluminum oxide forms thereon. While pure aluminum possesses a melting point of 660xc2x0 C., the melting point of aluminum oxide lies above 2000xc2x0 C. The mechanical cohesiveness of this thin oxide layer is, at 660xc2x0 C., so high that the melted aluminum is constrained from flowing outwardly and, for this reason, is not capable of establishing contact solely under the influence of gravity (with, e.g., a switch contact) in the same manner as a noble metal.
In view of these problems that arise, it would be desirable to provide a cost-favorable calibration element which avoids the above-noted disadvantages.
The present invention provides a solution to the challenge of providing a calibration device which permits an exact temperature calibration with base metals as well as noble metals at, in particular, lower temperatures.
The inventive solution differs from the heretofore implemented fundamental conceptxe2x80x94namely, the fundamental concept based upon movement of a calibration metal, once it melts as it reaches its melting temperature, solely under the influence of gravity to effect the opening or closing of a switch circuit. The inventive solution makes use of the change in volume which occurs during the phase change between the liquid phase and the solid phase of the metals, in that the inventive solution uses such metals as the means for opening or closing a switch contact of an electrical circuit. The forces which occur in connection with such a change in volume are substantially greater than the force of gravity. If one takes notice of the fact that, in an admittedly abnormal reaction, frozen water is in the position, without further assistance, to break a thick-walled glass bottle, it is easy to understand that melting aluminum is in the position, without further assistance, to bring to bursting its top surface thin-walled oxide coating.
In this connection, it is particularly advantageous, in connection with the present invention, that the establishment of contact with the switch contact requires a movement. During this movement itself, the tendency of the newly exposed aluminum top surface to form new oxide layers at the border surfaces with the air is at its lowest, whereby a secure establishment of contact is ensured.
In contrast to the continuous yet weak volume expansion which occurs over the entire temperature range, during a melting process there occurs a sudden and substantially stronger change in volume which, with respect to most metals, is an order of magnitude in the range of several percent of the volume. Aluminum is, in this regard, a special case with its volume increase of 7% and is, for this reason, especially well suited for configuring an inventive calibration device.
In connection with the present invention, it is especially advantageous if the housing of the calibration device possesses a resistance to pressure which is greater than the bursting pressure which effects the destruction of the oxide layer of the melt element during the melting process. The melt element, which can, for example, be formed of aluminum, experiences during a melt process an expansion of its volume. The pressure resistance of the housing causes the housing to channel the melt element, as it undergoes a volume expansion, toward a counter-contact so that, thereat, the pressure rises until it is larger than the top surface tension of the oxide layer. The oxide layer thus breaks apart so that the metallic aluminum is free to move. The invention is not limited to use of aluminum and/or silver. Rather, in lieu of these metals, suitable meltable metal coatings can be deployed, it being taken into account that such metals provide a precise melting point. Eutectic coatings comprise a precise melting point as such coatings have identical solid and liquid temperatures, as do pure metals.
There are also metals such as, for example, antimony, by which, during the melting thereof, a negative volume change occurs similar to that which occurs with respect to the anomalous behavior of water.
Additionally, such metals can also find use in the spirit of the present invention, since the inventive calibration device can be deployed for indicating decreasing or sinking temperatures as well as rising or increasing temperatures. In this event, instead of the establishment of a contact, the loss of a contact is produced and vice-versa.
Further advantages, details, and features are described in the hereinafter following description of five embodiments of the inventive device.